Preparation And Performance Study Of Fe_0.68Ni_0.32 Alloy As Inert Anode For Aluminum Electrolysis

In this paper,we prepared Fe0.68Ni0.32 alloy by medium frequency induction furnace.The XRD analysis showed the alloy phase is(Fe,Ni)and the SEM analysis showed that chemical compositions distribute uniformly but with a small amount of Fe2O3 and FeS impurities.Thermogravimetric experiment was carried out with Fe0.68Ni0.32 alloy suspending in distilled air at 970 ? for 22 hours.In oxidation process,a dense oxide film was formed at the alloy surface that will block the diffusion of oxygen atoms from atmosphere to alloy substrate.Thus,it significantly reduces the alloy oxidation.The result showed the oxidized kinetics behavior of the Fe0.68Ni0.32 alloy at high temperature follows parabolic rule,oxidation kinetic constant is 9.57 X 10-4Kg2·m-4·h-1 and the weight gain is 42.98mg/cm2.Oxidation experiments of specimens were carried out in room atmosphere at 970?for different times,then cooling in furnace.The phase analysis showed the outermost oxide is Fe2O3,the connection of oxide layer and alloy substrate decrease with oxidation time.The alloy oxide layer can be roughly divided into two layers:the outer layer of Fe2O3 and inner layer of Fe-Ni-O.The increase of oxidation time will results an increase in the oxide layer thickness,the Fe-Ni-O layer thickness and the depth of oxygen atoms immersed in the alloy.Hot corrosion tests were conducted in a mixed electrolyte of 3.5wt%Al2O3-4wt%CaF2-5wt%LiF-CR2.2 at 970 ?.It showed that the hot corrosion rate of alloy is 3.22mg/cm2·h.At the same temperature,electrochemical experiments in the electrolyte system of 3wt%CaF2-9.7wt%Al2O3-CR2.9 indicated that the open circuit potential of the alloy is 1.21 V,the corrosion potential is 1.60V,the corrosion current is 3.09×10-2A and the corrosion current density is 4.24×10-2A/cm2.Using Fe0.68Ni0.32 alloy as inert anodes,the high temperature transparent experiment was also carried out.Before the experiment,the alloy was oxidized for a period of time.During electrolysis,the anode generated a swarm of small bubbles at the bottom surface.Meanwhile,a wood stick with spark splint was put at the top of the furnace and it burned again.This proved that the generated gas is O2 and the anode is an inert anode.The cell voltage stabilized at around 5.5V,6.2V,6.8V and 8.8V at the anode current densities of 1.1A/cm2,1.3A/cm2,1.5A/cm2 and 2.0 A/cm2 respectively.The bubble foams released from anode bottom and cannot to form a covering large bubble.This is significantly with the bubble behavior at an industrial carbon anode.The gas bubbles were found to nucleate on the industrial carbon anode surface and undergo spherical growth,lateral spread,mutual impingement,and coalescence to form a larger bubble covering the bottom of the anode.On the industrial carbon anode,the bubble releasing period and coverage decrease with the increase of current density.Fixed current electrolysis was conducted in the electrolyte system of 3.5wt%Al2O3-4wt%CaF2-5wt%LiF-CR2.2 at 950? for 9h.Throughout the electrolysis process,lots of small bubbles were generating in a continuous manner from the anode surface.The anode size did not change after electrolysis.The purity of aluminum production was up to 97.11 wt%.Impurities were mainly iron(1.71 wt%)and silicon(1.08wt%);Nickel content is 0.005wt%;other impurities is less than 0.1 wt%.The current efficiency reaches 63.35%.Meanwhile,we developed a new high temperature electrolysis transparent cell for better view of a bigger anode bottom.